The present invention relates to liquid projection apparatus in the form of what is known as a xe2x80x98face-shooterxe2x80x99 array.
In the ink-jet art there are many liquid projection devices that utilize the acoustic resonance of capillary channels or chambers, hereinafter collectively termed xe2x80x98cellsxe2x80x99, associated with nozzles to provide a pressure wave to cause liquid to be expelled from those nozzles. These technologies are limited in their maximum frequency of operation by the liquid acoustic resonant frequency of these cells. In addition to this, the cells act as a restriction to flow causing pressure to be developed within the cells which effects drop ejection. Flow through the cells is therefore limited by the refill rate, causing a further upper limit to the operational frequencies of such devices. Moreover, the cells act as a trap for air bubbles and contaminant particles that severely disrupt operation and which are problematic to remove. Structures employing cells are also thereby restricted to handling liquids of particular rheology, high purity and stability. For example, unstable suspensions that are used to form white, gold and silver inks cannot be applied reliably with devices employing cells.
Further technologies described in the art provide excitational members in the bulk liquid in close proximity to the rear of a separate nozzle-plate. This configuration has the advantage of allowing bubbles to escape, but the method is intrinsically inefficient in use of energy and prone to crosstalk.
A further difficulty associated with printheads known in the art is that their construction is based on sub-assembly onto a three dimensional structure rather than onto a substantially two dimensional workpiece. This has the consequences of increasing the variability of the product and decreasing the manufacturing yield.
In the ink-jet industry, there is an unfulfilled requirement for printheads that can be fabricated with a sufficient number of ejection sites on a single printhead to constitute a page-wide array. The problem of producing such a printhead is the requirement that the fabrication processes must provide a structure with a high degree of consistency between thousands of ejection sites. Predominantly in the prior art, for example EP0728583A2, constructions are taught wherein there is a requirement to locate a number of components in three dimensions to form a linear drop on demand ink-jet printhead. This construction does not achieve the integration of the transducer means into a substantially planar form. It is the belief of the authors of the current invention that this lack of integration is the prime limitation on the width of the array that can be constructed by methods in the prior art.
The same construction difficulty arises in WO-A-94/22592 wherein excitation means are bonded to, but not integrated with, a material layer in which nozzles are formed. This prior art also requires an extended structure behind the nozzle-plate to provide the acoustic energy to enable drop production. The fabrication of this prior art must also proceed by fabrication in three dimensions, again introducing problems of component alignment.
As stated above, the inventors believe the solution to this problem is to understand that the extended construction can be achieved by integrating the transduction means to the surface. In practice, achieving this is not a trivial problem to overcome.
For example, if the structures disclosed in WO-A-94/22592 are reduced in height to achieve the substantially planar condition, a contradiction results. Firstly, the motional action of the PZT (piezoelectric lead zirconate titanate ceramic) as configured, reduces markedly with PZT height reduction. Secondly, the structure requires the surface to flex, but the PZT must remain rigidly bonded to a further surface. The structure cannot be reduced to a layer.
A second approach is to apply an annular ring geometry, such as is described in our EP-A-0615470, to form a surface array of such devices. If the flexural rings are set out and connected in an array there is first a problem of scale; the separation of nozzles on the array will be determined by the minimum achievable PZT ring outer diameter that will produce droplets at an acceptable drive voltage. This will be too large to form a high resolution linear array (such as 150 nozzles per inch, as is required, for example, in many printing applications). An attempt to apply the vibration of a surface (bimorph) flexural ring is disclosed in JP 09-226111. However in this case the rings being bonded to, or formed in, a material layer are unavoidably coupled around the outer circumference to the material layer as a whole, inducing undesirable crosstalk between rings.
An alternative form of excitation, from a similar physical structure to JP 09-226111, is shown in JP 10-58672. In this patent application, the radial vibration of a surface ring is apparent. Again, however, the rings are unavoidably coupled around their outer circumference to the material layer as a whole, inducing undesirable crosstalk.
A subsequent patent application, JP 10-58673, also discloses an annular ring geometry employed to produce a surface meniscus resonant wave. The inventors of JP 10-58673 seek to improve fluid coupling by introducing a further structure at a defined depth of ink beneath the nozzle, forming a flow constriction which effectively defines a resonant cell structure, and thereby the substantially planar condition of construction is lost.
In the prior constructions taught in JP 09-226111, JP 10-58672, JP 10-58673, the droplet formed is small compared to the xe2x80x98nozzlexe2x80x99 opening of the substrate. The relatively large nozzle is correspondingly more sensitive to undesirable wetting of the front face of the printhead than the constructions in which an issuing jet fills the nozzle to produce a droplet of similar size, for example under conditions of ink having low surface tension or physical shock to the printhead. This sensitivity arises from the lower pressure differences that the relatively larger diameter meniscus within the larger xe2x80x98nozzlexe2x80x99 can sustain.
According to the present invention there is provided a device for projecting liquid as jets or droplets from multiple nozzles, the device comprising;
a plurality of transducers oriented substantially parallel to one another and each having an inner face and an outer face opposite said inner face, the transducers being arranged in a substantially planar array;
a plurality of nozzles, each nozzle being associated with a respective transducer which is excitable to cause movement of the associated nozzle in a direction substantially aligned with the nozzle axis, to project liquid therefrom;
liquid supply means for supplying a liquid to said inner face of said nozzles;
means for selectively exciting transducers as required, thereby to project liquid as jets or droplets from the respective outer face by movement of the liquid through the nozzle in response to the movement of the nozzle.
Thus, in such a device, the transducers are all aligned in the same direction and, where the transducers are rectilinear, they all have a major axis parallel with that of the other transducers. Even where the transducers are not rectilinear, as long as they are congruent, they will have at least one edge in parallel with the same edges of other transducers in the array.
By the term xe2x80x9ctransducerxe2x80x9d is meant a local region of the liquid projection device that can be stimulated into motion by an associated individually-addressable excitation means. By the term xe2x80x9csubstantially planarxe2x80x9d it is meant that the height of the components is small with respect to the lateral extent of the array of individual components.
The inventors believe the key to achieving a page-wide array is making the array in layers which may be aligned optically using surface processing techniques.
The transducer components may be formed in one piece comprising, for example, a piezoelectric or similar excitation means. The transducers may also be formed as a composite component in which an excitation means is, for example, bonded to or formed integrally with one or more other material bodies which may, for example, provide a mounting support or substrate for the excitation means.
It is not necessary that all transducers have nozzles associated with them. However, for those transducers that do have associated nozzles, the nozzles may pass through the exciting means or through the material body (or bodies) that, together with the exciting means forms the composite transducer or through both the exciting means and that material body (or bodies). In each case the transducer surfaces that each nozzle intersects in passing therethrough define the inner and outer faces of that transducer. Correspondingly, in this specification, implementations of the invention in which a nozzle (or nozzles) is formed in a separate component from that to which excitation is directly applied are together considered to comprise the transducer.
Preferably the excitation means, and associated material body (or bodies) if used, that form the transducer are in the form of layers. Forming the transducers of the projection device from layer components in this way allows accurate registration of their component parts to be more easily and reliably achieved in assembly of the liquid projection device than is achieved with the three-dimensional constructions prevalent in the prior art.
Distinct transducer regions may be formed within the material layer by selective thinning of the layer, allowing the respective region to move with reduced constraint from the remainder of the material layer and thereby enhancing transducer operation. A further reduction in constraint can be achieved by slitting right through the material layer to form slits around each transducer region. The regions may thus be in the form of beams formed by slits within or through the material layer, and each of the slits may be sealed. Furthermore, the slits may be arranged in the form of a comb or in the form of two interdigitated combs.
In the prior art slits were included simply to enable bending to take place and the have to be thin otherwise they will leakxe2x80x94so they are a mixed blessing. We have turned the problem to our advantage by using the slits not only as a means of getting the bending, but also as a method of suppressing crosstalkxe2x80x94so the slits become isolation means decoupling the transducers. The isolation can be improved by filling the slits with a compliant medium, and this overcomes the leakage problem. The slits may be of comparable width to the transducer as the compliant medium allows the separation to be chosen to get the isolation needed.
Decoupling is thus achieved by separating the transducers by substantially parallel gaps within or through the surface, with the principal constraint and contact of the transducers to the bulk of the material layer adjacent to the distal ends of those gaps, and preferably with the greatest amplitude of transducer motion proximal to a nozzle opening (preferably located distant from those distal ends) which ejects the liquid (typically ink). Further, the physical separation allows a second material to be used to fill the space between the transducers. If this is chosen to be a compliant rather than a stiff medium, excellent decoupling can be achieved. Alternatively a compliant sealing layer may be used to seal the space, again maintaining excellent decoupling. The use of slits to divide flexural nozzle-plates is disclosed in WO-A-94/22592, but a planar array of transducers bearing nozzles is not taught, and the width of the slits is restricted by the tendency of the liquid to seep or be pumped to the outer surface during filling and drop firing respectively. In the present invention, nozzle-plate motion is induced by flexural, rather than extensional, elongation of rigid rods, as in WO-A-94/22592 Therefore, in the present invention, the mechanical properties, e.g. stiffness, of the nozzle-bearing layer are comparable to those of the xe2x80x98excitation meansxe2x80x99 layer, helping to maintain co-planarity of adjacent nozzle-bearing transducers. This helps both to prevent liquid egress through unsealed slits and, in the case of sealed slits, helps retain motional excitation to produce droplets with low or acceptable levels of crosstalk carried by the sealing means. The use of sealing means for liquid sealing without introducing substantial crosstalk is therefore one aspect of the current invention.
The layer-surface may be utilised as part of the transducer means, selectively decoupled from the extended material layer to suppress crosstalk and configuring the excitation means within this transducer to function jointly in the bending mode. Further, this new layer-surface approach allows the use of nozzles of dimension smaller than the diameter of ejected droplets (and with good resistance to blocking in the case of suspension liquids such as pigmented inks), so avoiding the sensitivity to xe2x80x98wetting-outxe2x80x99 shown by the prior art devices.
In one construction arising from the invention the transducers may comprise three material layers, each optimized for their function, for example; a first layer of piezoelectric material providing the excitation means and which is mounted on a second support layer of (for example) stainless steel sheet that cooperates with the piezoelectric layer to provide flexure and which in turn carries on its opposite face a third thin polymer layer, in which the liquid-ejection nozzles are formed. Alternatively, such functions may be combined into two or even one layer.
For those transducers bearing nozzles, we term the local vicinities of the nozzles of those transducers as the xe2x80x98nozzle regionsxe2x80x99. In use, liquid present at a nozzle region at the inner face of a transducer passes through the nozzle to be projected as a jet or droplet (or plurality of jets or droplets) when the transducer is excited such that at least the nozzle region moves (with appropriate amplitude and response time) in a direction substantially aligned with the nozzle axis. Most conveniently both the nozzle axis and the motion of the nozzle region is in a direction substantially parallel to the surface normal of the nozzle-bearing region of the transducer.
The nozzles within the nozzle bearing regions are arranged in an array within the device, which array may be one-dimensional such as a row or a line of nozzles, or may be two-dimensional such as multiple rows or lines preferentially arranged parallel to each other. Such a nozzle array ensures that there is an array of, at least, nozzle-bearing transducers. In addition, within that array may be additional transducers without nozzles (for example interspersed with the nozzle-bearing transducers). These additional transducers can be helpful in suppressing residual crosstalk-induced layer resonances and layer edge effects.
In preferred embodiments of the invention at least those transducers bearing the nozzle-bearing transducers are individually addressable. It is usually desirable that the motion of one nozzle bearing transducer (excited to eject liquid from its corresponding nozzle(s)) does not cause comparable motion of other nozzle-bearing transducers or substantial pressure fluctuations in liquid adjacent nozzle-bearing regions of other transducers. In this way, not only are the nozzle-bearing transducers individually addressable, but in addition individual control of liquid projection may be obtained from each nozzle-bearing transducer and, where each such transducer includes only one nozzle, individual control of ejection from each nozzle is obtained. We refer to this generally as reducing xe2x80x98crosstalkxe2x80x99 between nozzles (and/or nozzle-bearing transducers).
The array of transducers referred to above, and/or any of their constituent parts (such as excitation means, support body for excitation means and/or nozzle-bearing body) may be integrally formed with one another or else may be individually formed. If integrally formed then, to reduce crosstalk mediated through the solid elements of the device (xe2x80x98mechanical crosstalkxe2x80x99), it is generally desirable to separate or partially separate them by gaps, typically in the form of slits. The gaps may extend through one, several or all component layers of the transducers (for example through just the exiting means and its support body but not through a thin polymer nozzle-bearing layer).
In the case that the slits or gaps extend through all those components that otherwise provide a slit between inner and outer face of the transducer it is generally advantageous to seal those slits or gaps against liquid egress or evaporation. This may be done, for example, by incorporating into the slit a soft elastic material body that is poor at transmitting transducer motion parallel to the slit, such as Latex Solder Resist as supplied by RS, part number 561549. Alternatively a further material layer (or layers) (which can be considered, to the extent that it contributes to transducer operation as a further transducer component; and which can be considered, to the extent that it influences only overall device performance, as a common component for all the transducers that it seals in this way) can be applied across the slits thereby to seal them. This further material layer may be formed of, for example, 25 micron polyimide sheet such as Upilex.
Therefore, in favoured implementations of the present invention, there is provided a material layer or layers bearing nozzles and which are excited into motion with respect to bulk liquid brought to them. This action induces pressure excursions in the bulk liquid in nozzle regions of transducers. Each transducer bearing a nozzle is individually excited into motion (xe2x80x98individually addressablexe2x80x99), and the invention allows simple construction of such individually addressable multi-nozzle droplet dispensing devices.
This invention aids the reduction of mechanical crosstalk between nozzles, thereby aiding individual control of liquid ejection from each nozzle as well as providing individually addressable transducers.
Devices according to the invention may with advantage provoke both positive and negative excursions of such liquid pressure at least in nozzle-bearing regions to project liquid from the said nozzle. The xe2x80x98motional nozzlexe2x80x99 method does not rely upon low compressibility of the liquid or upon stiff cells and so contrasts with conventional ink-jet droplet dispensation apparatus wherein pressure is generated compressively within the cells.
The current invention provides xe2x80x98directxe2x80x99 excitation of the nozzle region (xe2x80x98directxe2x80x99 in this sense meaning that the excitation is not primarily transmitted to the nozzle region by using the liquid as the transmission medium. Rather, it is primarily transmitted via the solid material components of which each respective transducer is formed. In this way a device according to the invention causes the greatest pressure excursions to be produced in the nozzle-bearing region immediately behind the nozzle, this advantageously thereby reduces xe2x80x98liquid crosstalkxe2x80x99 mediated by the liquid. By xe2x80x98liquid crosstalkxe2x80x99 we mean energy transfer via the liquid from the nozzle region of one transducer to the nozzle region of another transducer (otherwise, potentially making an undesirable contribution to liquid ejection also from that other nozzle).
A unique advantage of devices according to the present invention over devices in the prior art is that the individual addressability and commonality of substrate of the transducers advantageously permit active cancellation of residual crosstalk signals from one local region by the partial or phased (or both) activation of neighbouring transducers. This results in the ability to operate next nearest neighbour local regions synchronously, using the intervening transducers (which may have no nozzles) to actively damp crosstalk.
The integration of the excitation and nozzle means and the motional excitation mechanisms also reduces or eliminates the need for separate liquid cells for each nozzle. In turn, this suppresses the sensitivity of jet or droplet projection to bubbles in the liquid which, in cell-based designs, can become trapped in those cells to provide continuing perturbation of jet and/or droplet projection.
The current invention also allows the high accuracy components to be concentrated in, at most, a few, sheet-like layers. This allows ease of fabrication since the piece parts are assembled onto a single plane.
The liquid projection apparatus described herein is believed by the inventors to be unique in the ability to deposit, white, gold and silver inks, and other inks of large pigment size and unstable dispersion characteristics, due to the unique ultrasonic action of the transducer means.
In addition, the action of the motional drive of at least the nozzle regions allows the device to perform an ultrasonic cleaning action of at least those regions of the transducers, including the inner and outer faces of those regions and of the nozzles themselves. This allows maintenance with reduced need for purging and wiping of the face of the device.
Advantageously, the excitation of the transducer can substantially localise the stimulation pressure to that liquid directly in contact with the nozzle region. This can be achieved by, for example, making the nozzle region less stiff to bending motion than the rest of the transducer, so that the greatest motional response (and thereby the greatest stimulation pressure) occurs in the nozzle region itself.
This means that in the new liquid projection device a very sharp resonance is not necessary and the liquid projection is thus further less sensitive to factors such as the consistency of the liquid, the presence of air bubbles in the liquid and manufacturing tolerances of the device, which generally have a very serious effect on the performance and cost of a conventional liquid projection device. The new liquid projection device is therefore potentially cheaper and more reliable in operation than the prior art and does not require such complicated liquid conditioning apparatus.
Advantageously, the thickness of each transducer in the motional direction satisfies the inequality:       ∑          (                        t          i                          c          i                    )         less than       1          2      ⁢      f      
where t, is the thickness of the rth layer of material in the transducer and c, is the speed in that layer, at the operating frequency f, of either compressional or shear waves propagating the layer in the direction of its thickness.
Other excitation means than piezoelectric elements suitable for use with the invention are electrostrictively, magnetostrictively and electrostatically deflected electromechanical elements.
In one example embodiment, piezoelectric elements are used as the excitation means, exciting motion of material layer(s) bearing nozzles responsive to an electric field applied to those elements. The elements are in the form of thin layers of piezoelectric material having electrodes on opposing faces. When pre-formed as fired elements, one such face of each piezoelectric element is mechanically bonded to a part of the nozzle-bearing material layer. When a refractory nozzle-bearing layer material (such as ceramic) is used, the piezoelectric elements may alternatively be deposited as a thick film (for example by screen printing) and fired in situ to form the excitation means. In both cases the piezoelectric layer is arranged to expand or contract on application of a voltage to it. Thus each element in combination with that area of the nozzle-bearing material layer to which it is bonded cooperatively form a transducer in the form of a flexural member. A nozzle formed either in this bonded area or in a nearby area of the nozzle-bearing layer thereby completes formation of a nozzle-bearing transducer. Both nozzle-bearing transducers and non-nozzle-bearing transducers can thereby be excited into bending motion substantially perpendicular to the electroded surfaces of the piezoelectric element and of the transducer as a whole. This provides, as a first advantage, motional excitations of the transducer and the nozzle-region within it, in a simple and efficient manner.
A second advantage resulting from this embodiment is that the exciting part of such a nozzle-bearing transducer structure (in this case the piezoelectric element and the region of the nozzle-bearing material layer to which it is bonded) can have a much lower acoustic impedance than in a conventional liquid projection device, the acoustic impedance of this exciting part being comparable to that of the nozzle region (and the same impedance if the nozzle lies within the exciting part). These circumstances mean that the amount of excitational energy stored in such a transducer is smaller than that stored in conventional devices and that a larger amount of energy can be transferred during excitation in either direction between the exciting part and the nozzle region. This makes it possible to control the excitation of the nozzle region directly by feeding appropriate drive signals to the excitation means, so allowing unwanted motions to be actively suppressed.
The isolation of one transducer from another (reduction of crosstalk) may be improved by fabricating transducers separated by gaps, or by locally removing material to form gaps within layers that are common to other transducers, particularly other adjacent transducers, thereby reducing the mechanical coupling between them. This may be achieved, for example, by grinding or by laser cutting, the latter advantageously allowing the fabrication of narrow slots of approximately 5 microns width, producing well-defined slits, free of blade xe2x80x9crun-outxe2x80x9d.
The transducer may include an additional substrate (preferably, but not necessarily, in the form of a layer) on which said excitation means is mounted, said substrate having an aperture, and a flexible membrane mounted on said substrate and covering said aperture, wherein said nozzles extend through the region of the flexible membrane that covers said aperture. In such a construction employing a separate substrate and flexible membrane, the flexible membrane may be bonded to the substrate which may be formed of, for example, stainless steel.
In many applications of the present invention it is desirable to arrange the multiple nozzles in an array (or series of arrays) each being individually addressable by the excitation means of its respective transducer. These arrays of nozzles may be formed to define a common outer surface which surface may conveniently be coincident with the outer face of a common nozzle-bearing layer. In this case, preferably, the shape and location of the, or each, excitation means and transducer is arranged to avoid generating travelling waves which transfer energy between transducers from one nozzle to another, so mechanical crosstalk is minimized. This may be achieved for example by forming slits (described above) in the nozzle-bearing layer and/or in any auxiliary material layers employed.
Further refinement may be achieved by providing sensing means separate to or integral with the excitation means of nozzle-bearing transducers and by using feedback from the sensing means to nullify the background noise. Similarly, transducers without nozzles may alternatively or additionally, be excited to provide motional or pressure damping or cancellation in the nozzle regions of nozzle-bearing transducers. For this function such transducers without nozzles may beneficially be interspersed between nozzle-bearing transducers to form an alternating array. Indeed, benefit may be found even by the provision of simple xe2x80x98passive elementsxe2x80x99 between nozzle-bearing transducers, these xe2x80x98passive elementsxe2x80x99 typically comprising nozzle-less transducers that also have no excitation means or no drive provided to any excitation means that they do possess.
The nozzle-bearing layer may be mounted on a manifold having a cavity for supplying ink to at least the nozzle regions. The manifold may include excitation damping materials or otherwise be designed to inhibit resonance and, by extending across all or several nozzles can avoid the xe2x80x98cellxe2x80x99 construction of most conventional ink-jet printheads with their associated sensitivity to air bubbles and solids deposition.
The liquid projection device may also be formed as a piecewise fabrication of preformed components. This advantageously allows the choice of boundary conditions for the transducers, the pre-testing of component parts and the employment of further layers for nozzle regions and sealing structures between transducers.
Preferably, the device includes an electronic drive coupled to the terminals and hence to the transducers, and arranged to provide drive signals independently to respective transducer terminals, whereby production of droplets from the nozzles is achieved selectively by corresponding selective generation of drive signals.